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chromium/third_party/blink/web_tests/fast/canvas/canvas-textMetrics-width.html 

<!DOCTYPE html>
<body>
<p>On success, the red text "Hello World" should be tightly contained inside the two thin red lines.</p>
<canvas id="canvas" width="300" height="50"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var text = "Hello World";
ctx.font = "50px serif";
ctx.fillStyle = '#FF0000';
ctx.textBaseline = "top";
ctx.fillText(text, 1, 0);
var textMetrics = ctx.measureText(text);
ctx.fillRect(0, 0, 1, 50);
ctx.fillRect(Math.ceil(textMetrics.width), 0, 1, 50);
</script>
</body>